Automatic telephone-exchange system and apparatus.



Nu. 709,740. Patentad Sept. 23, I902.

' A. M. BULLARD &. M. C. ROBTY.

AUTOMATIC TELEPHONE EXCHANGE SYSTEM AND APPARATUS. (Application filed Aug. 29, 1901.)

um Model.) s Shocks-Sheet W1 ZYVESSES U iuw' No. 709,740. atented Sept. 23,1902 A. M. BULLABD & M. C. RO'RTY.

AUTOMATIC TELEPHONE EXCHANGE SYSTEM AND APPARATUS.

(Application memml fae, 1901.

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1 III WITNESSES E News pznms m. Pno oumu. WASHINGTON o c I No. 709,740. Patented sept. 23, I902.

A. m. BULL-ARD & M. c. RORTY. AUTOMATIC TELEPHONE EXCHANGE SYSTEM AND APPARATUS.

(Application filed Aug. 29, 1901.]

(N0 Mudel.)

6 SheeisSheei 3.

No. 709,740. Patented Sept. 23, I902.

A. M. BULLARD & M. C. ROHTY. AUTOMATIC TELEPHONE EXCHANGE SYSTEM AND APPARATUS (Application filed Aug. 29, 1901.)

(No Model.) 6 Sheets-Sheet 4,

. ||nl\ 2 f r M Patented Sept. 23, I902. A. M. BULLARD & M. C. RORTY.

AUTOMATIC TELEPHONE EXCHANGE SYSTEM AND A PPABATUS.

(Application filed Aug. 29, 1901.)

(m Modal.) e Sheets-Sheet 5.

Inventors m: mmms Farms cc.. vumoumcm. \VASHNGTON, w. c.

No. 709,740. Patented SBpt. 23, [902. A. M. BULLARD 15. M. C. RUBTY. AUTOMATIC TE LEPHONE EXCHANGE -SYSTEM AND APPARATUS.

(Application filed Aug. 28, 1901.)

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il ivrrnn STATES Parent Orrtcn.

ALBERT M. BULLARD, OF SOMEEVILLE, AND MALCOLM C. RORTY, OF DEDHAM, MASSACHUSETTS, ASSIGNORS TO THE AMERICAN BELL TELEPHONE COMPANY, OF BOSTON,

TION OF MASSACHUSETTS.

MASSACHUSETTS, A CORPORA- AUTOMATIC TELEPHONE-EXCHANGE SYSTEM AND APPARATUS.

SPECIFICATION forming part of Letters Patent No. 709,740, dated September 23, 1902.

Application filed August 29. 1901. Serial No. 73,742. (No model.)

T0 at whom, it ntcty concern.-

Be it known that we, ALBERT M. BULLARD, residing at Somerville, in the county of Middlesex, and MALCOLM O. RoRTY, residing at Dedham, in the county of Norfolk, State of Massachusetts, have invented certain Improvements. in Automatic Telcphone Exchange Systems and Apparatus, of which the following is a specification.

This invention constitutes an improvement in automatic telephone-exchange apparatus and circuits,and has especial reference to antomatic telephone-exchanges designed to supply facilities for intercommunication in small communities where all of the possible subscribers may be accommodated by a small numberof substation-circuits, some or all of which may be party or polystation circuits.

In organizing an effective automatic telephonic-exchange apparatus it is necessary to provide electrical or electromagnetic mechanisms and appropriate circuit arrangements at the central or switching'station controlled for the exercise of their Various functions by coordinate operative devices at the subscribers stations and combined for the orderly performance of the several distinct suboperations involved in the general work of the exchange. These suboperations in all such organizations comprise the selection by a substation of the line of any other substation with which communication is desired, the testing 01' such line to determine whether it is at liberty or is already engaged, the connection of the two substation-oircuits if the one wanted is found to be disengaged, the transmission of call-signals to the wanted substation, the establishment of intrusion-forestalling devices in association with liothof the two united substation-circuits, whereby a third substation desiring to communicate with either is prevented from establishing the requisite switch connection and from interrupting the operations of either of the engaged circuits, the associating of a potential busy or engaged signal with a line or lines already in use and enabling the same to become active and manifest itself to a third circuit desiring interconnection with such ments of the said circuits.

line or lines, thus indicating that for the present the desired line is engaged and that access cannot be had thereto, and, finally, the disconnection and restoration to the normal condition of two lines which have been connected with one another.

Furthermore, in automatic exchanges comprising party-lines or polystation-circuits it is likewise necessary to provide facilities whereby any station may ring up or send callsignals to the other stations on the same circuit, means for protecting any such polystation-circuit from intrusion while any two stations thereof are communicating and Where by such a circuit when tested by any other at the central station is made for the present inaccessible, and means for announcing or transmitting the busy signal to the station endeavoring to establish connection therewith as effectually as if the two stations in communication were thoseoftwo diiierentcircuits switched together at the central station,

In an application for patent, Serial No. 26,441, filed by us August 9, 1900, is described an automatic telephone-exchange sys tem comprehendinga numberof metallic telephone-circuits extending from substations, respectively, to a central station where is located a common source of electromotive force supplying electrical energy for all require- Each circuitat the central station has a movable terminal member and a plurality of fixed terminals, these being so relatively arranged that the movable member of each circuit has within its range one of the fixed terminals of all of the other circuits and may by appropriate operation be brought into cont-act with any desired one of them,thereby uniting the twolinesconcerned. In the said system the movable terminal of a circuit is brought into line with the fixed member of another circuit,with which communication is desired, by a series of successive interruptions or intermissions of current corresponding to the number of the line Wanted, produced by a suitable periodic circuitbreaker at the substation initiating the call in one of the conductors of the circuit, and is then brought into actual contact therewith by the establishment of a steady current through both conductors of said circuit, both of these suboperations being carried out by electromagnetic mechanism controlled. by relays are provided whereby both lines are madein-' accessible toall others while engaged with one another.

The automatic exchange of our present invention employs a central and universal battery or like source of electromoiive force furnishing all of the current necessary for the accomplishment of the selecting and connecting processes involved in uniting any two'substation-lines for through communication and also the current required as a supply for the telephone-transmitters of the two substations concerned in talking over such a through or compound circuit; but the calling-current is preferably developed by magneto electric generators at said substations.

In some respects this invention may be regarded as an improvement upon that of the said joint application, since both concern automatic telephone-exchange circuits and apparatus and since some features of the said application are also made use of in this invention. The invention, however, involves certain other features both in circuits and appliances which are ditl'erent not only from anything disclosed by the said joint invention, but also from anything which, so far as we are aware, has thus far appeared in the art as represented by other inventions of the same general class, while such devices as those to which reference has been made as being employed both in the said joint invention and in the present invention have generally in the latter different circuit arrangements and new cooperative relations with the said different appliances.

Ourinvention realizes a simple but exceedingly effective and reliable automatic telephone-exchange apparatus for a limited nu mber of main orsubstat ion circuits which is particularly well adapted for polystation-lines and which perfectly meets the several requirements hereinbefore recited. Its substationcircuits when arranged for-talking are metallic or consist of paired conductors through- 0utthat is, through the switching mechanisms as well as in the line portionsand the selecting and connecting devices of-each circuit comprise two movable or active terminals of said circuit representing, respectively, the two main conductors thereof and adapted to move into juxtaposition to fixed or passive terminal contact members successively of all other circuits of the exchange arranged in pairs and to make contact with the pair of points constituting the fixed terminal members of the circuit with which communication is desired when that pair is reached. The selective motion of the movable terminals is due to successive interruptions or intermissions of the main-line current acting through appropriate electromagnetic-motor mechanism, the said interruptions being produced by the operation of any suitable periodic circuit-breaker at the substation making the selection, and the connecting motion thereof is dependent upon the operation of other electromagnetic mechanism irresponsive to the selecting-current interruptions, but responsive to the establishment in the circuit of a steady current, which occurs when the circuit-breaker having ceased to operate the circuit is closed through the substation telephone apparatus.

We employ ordinary magneto-generators andpolarized bells for sending and receiving call-signals, and at each station provide in association with said generator two keys, which are to be alternatively manipulated according as the bell to be rung is that of another station on the same circuit oristhat ofa station on another circuit con nectedthrough the central station. The call-bells of all stations have their magnets connected in earth branches from one side or conductor of their respective main circuits, a condenser being included in each such earth branch in the usual manner, andthe balance of the circuit is maintained by associating with each bell branch a corresponding earth branch from the other side of the circuit containing similarly a condenser and an artificial impedance of like value to that of the bell-magnet.

Our invention also comprises the'provision for each circuit of means for balancing on one side of the circuit the resistance on the other of the central-station relays controlling the selecting and connecting magnets and for regulating when two substations of unequal resistance are united to form a th roughcircuit the amount of current supplied to each. 1

It comprehends also means whereby when communication has been established through the central station between two substationcircuits the call-generator at the substation which has originated the connection when operated is enabled to ring the bells at the substations of the other circuit without at the same time ringing the bells or operating call-signal devices at the remaining stations of its own circuit. I

Another important feature of'our invention is an improved means of protecting both circuits concerned in forming a compound circuit for through communication from intrusion or from being interrupted or interfered with bya third circuit during the'pendency of such communication, the call-originating circuit receiving such protection from the outset of its selecting operation and the call-receiving circuit as soon as connection with its fixed terminals is established. This means involves the normal detachment of the movable terminals of each substationcircuit from the main conductors of such cir- ,on the common core.

cuit and their control by a differential testing or discriminating relay which if the desired line be disengaged is excited by the passage of current through but one of its coils and becoming operative acts as an electromagnetic switch to attach the said terminals to their corresponding main conductors,

but which should the line wanted be already of the wanted circuit. Moreover, when under the conditions of our present invention any two substation-circuits are united for through communication there is constituted an improved talking-circuit conductively continuous throughout.

The invention includes also various details of improved mechanism and circuit arrangement, hereinafter fully set forth, described,

and explained.

In the drawings which accompany and illustrate this specification, Figure l is a diagrammatic representation of the central-station connections and appliances of two substation telephone-circuits and the substation l apparatus and arrangement of one of them, the same indicating andi-llustrating the mode of operation of the exchange. Fig. 2 is a diagramof a polystation telepl'lone-circuit shown as extending between two substations and the central station, the complete electrical arrangement of such circuit at the central station and at both substations beingindicated in detail. Fig. 2 isadiagram of the compound talkingcircuit formed of two sub Fig. 3 is a diagram repre-. ,senting the apparatus and electrical connecstation-circuits.

tions of a single.substation-circuit at one substation and at the central station, the latter on a somewhat-enlarged scale to illustrate more clearly the mode of operating certain circuit-changing devices employed in the invention. Fig. 3 is a diagram of a portion of a calling and a wanted circuit, illustrating a modified construction and arrangement of the intrusion forestallingand preventingappliances. Fig. 3 is a diagram of the calling or ringing circuit formed by the connection at the central station of two SLlbSlifl'tlOIl-Cllr cuits before the telephone at the called station has been removed from its switch-support. Figs. 4, 5, 6, and 7represent different views of the selecting and connecting or switch mechanism at the central station of a single main circuit, Figs. 4 and 5 being, respectively, plan and elevation views thereof; Fig. 6 a horizontal section on the line w 00 of Fig. 5 looking down, and Fig. 7 an elevational view of the rear or right of Fig. 4 looking in the direction of the arrow. Fig. 8 is a detail of the movable or selecting terminal of the circuit apparatus shown in Figs. 4 and 5. Figs; 9 and 10 are respectivelya front elevation, partlybroken away, and a side elevation with a portion of the casing removed to disclose the mechanism of a periodic circuitbreaker suitable for use at the substations.

Like reference letters and figures refer to like parts wherever used.

The operation of the invention will bemost easily followed and understood by a consideration of the diagram of Fig. l,aided by occasional reference to the other figures. It is, however, to be kept in mind that the said diagram indicates the second circuit merely by the beginning or central station end of the main conductors thereof.

The circuits are illustrated by Figs. 1, 2, and 3, wherein 0 represents the central station,and L and Usubstation-circuits converging thereto, each from one or more substations. The circuitL is shown as being apartyline or polystation circuit leading by its main conductors 2 3 from two substations X and X The circuit L is assumed to extend outward by its main conductors 4 5 to a substation Y, and may of course also be extended to other substations.

Although two circuits only are indicated, it mustbe understood that in practice a'number of such circuits will enter the same central station from substations and that each isthere provided with its own individual selecting and switching mechanism, whereby it may by appropriate action taken at any of its own substations select and connect with any other circuit entering the central station. Each circuit for the general .purpose of being switched to another is furnished with one set of active or movable terminals, whose function it is to select and connect with fixed terminals belonging to the said other circuit when the switch connection is made in pursuance of its own action and with multiple sets of passive or fixed terminals, one set for and within range of the movable terminals of each and all of the other circuits, so that the said circuit may be selected and connected with by any other, pursuant to the action of the latter. t

The movable terminals of each circuit are carried by a bar centered in vertical arbors or journals at one end and capable of revolving or swinging at its other end in a wide are around said center, and this bar may be termed the selectonarm orthe selector.

Falk February 19, 1901.

The fixed terminals of the several circuits may also be termed fixed contactsor fixed points.

Central-station apparatus-For each substation-circuit there are at the central station two inductance or choking coils, a balancing resistance, a testing and discriminating relay, a controlling-relay, and a selecting and connecting mechanism governed by said controlling-relay and comprising a selectingmagnet, a retaining-pawl magnet, a connecting-magnet, a selector-arm and certain associated switches or circuit-changers, the said selector-arm and switches being under the joint control of said magnets.

Referring to circuit L, for example, I i represent the said two inductance-coils, which, as shown, are placed one on each side of the central source of electromotive force in a bridge, branch, or terminal loop 8 of the circuit joining the main conductors at points 6 and 7. In accordance with the accepted principles of central and common battery telephone-exchanges the office of these inductance-coils (of which apair is supplied for each substation-circuit) is to individualize the several substation-circuits or pairs of circuits supplied from the same source and while permitting the ready flow of current from the said source for sufficient transmitter-supply to obstruct and oppose a high impedance to voicecurrents which in transit between the transmitter of one station and the receiver of the other would'otherwise be liable, at least in part, to be short-circuited through the branch 8 containing the source S, which branch forms an actual bridge or cross-wire between the main conductors of the compound circuit formed of two substation-circuits united for conversation.

R and R are the controlling-relays of main circuits L L respectively. The controllingrelay, as indicated by the drawings, is preferably such a one as that for which Letters Patent of the United States No. 668,554 were issued to Albert M. Bullard and Louis A.

It has two armatures of diverse inertia. One of these, the light and quick-acting armature g, controls the local circuit 67, which contains the selecting-magnet and is capable of responding to rapidly intermitting or pulsating currents, and the other is the relatively heavy armature h, which is sluggish in its movement and responsive (by motion to its forward or active position) to a steady or sustained current only and which controls the local circuit heavier armature extends in both directions from the said central pole, while the lighter extends to one only of the end poles. The relay is wound with two exciting-coils 10 10 connected in series, together with the battery S, in the substation-circuit. The rearward arm of the sluggish armature h is slightly heavier than the other, and the armature is therefore normally over-balanced in that direction, being thus brought into close proximity to the rear magnet-pole and as far as possible from the forward pole. Thus when interrupted currents pass through the coils the light armature, acted upon by the forward coil only, responds readily to each intermittent current, while the heavier sluggish armature is held in its position of rest by the rearward coil; but, as will be explained more in detail, one of the consequences of the movement of the light armature is the establishment of a short circuit around the rearward exciting-coil. When this occurs, the heavy armature is no longer attracted at its hinder end and yields to the influence of-the forward coil and the end pole excited thereby. As long as rapidly-succeeding interruptions occur the transient shortcircuitings of the rear coil are so brief that the inertia of the sluggish armature prevents it from moving; but as soon as a sustained current flows through the forward coil the steady attraction of the light armature is promptly followed by the movement toward the forward pole of the heavy and sluggish armature. If desired, two relays might alternately be useda quick-acting one for selecting the wanted line and a sluggish oneforestablishingconnectiontherewith. So, also, the selecting and connecting magnets in the local circuits which are controlled by the relayR might be operated directly by the main-line currents. The forward exciting-coil 111 of the controlling-relays is shunted by a condenser 7L Rh in each main circuit is the balancing resistance thereof. It is concerned with the talking-circuit only, and its position therein balancing in one conductor thereof the coils 10 of the relays R R and the condensers shunting them in the other is best shown by the diagram Fig. 2. For this purpose the said resistance is preferably made substantially equal to the sum of the resistances of the said relay-coils. The said balancing-resistance likewise tends to regulate the supply of battery-current when two main circuits of unequal resistance are interconnected. Each line is supplied with current from the common source through the inductance or choking coils I l i i, and when one line is united to another each line obtains current through it own inductance-coils and also through the inductance-coils of theother line. It is evident that the introduction of resistance into the circuit between the points where the current-supply connections join their respective component circuits will decrease the amount of current delivered to each through the inductance-coilof the other. The resistance Rh thus becomes a regulator, operating to prevent a substation-circuit of ICC low resistance from depriving a circuit of l ofthem,e,through the substance of thehorsehigher resistance connected therewith of the current strength requisite for good transmission.

The electromagnetic motor mechanism for selecting a wanted circuit and for connecting therewith and the associated switches or circuit-changers besides being indicated conventionally by the diagrams Figs. 1, 2, and 3 are shown in mechanical detail in Figs. 4, 5, 6, and 7. There are as manysuch mechanisms at the central station as there are substation-circuits; but as they are all alike it is sufficient to describe the mechanism belonging to one circuit only. Z is a general framework supporting the several magnets and moving parts of the mechanism. To the plate Z of said frame is secured an areshaped block U of non-conducting material, in which are embedded a number of metallic strips projecting radially from the periphery thereof. These are the passive terminals or fixed contacts of the several other substation-circuits of the system and are arranged in pairs u U, each pair representing one circuit, there being for each mechanism at. least as many pairs of fixed contact-strips as there are other circuits in the system. In practice for the purpose of avoiding false starts of the selecting mechanism and to provide against the chance of the said selecting mechanism being unduly actuated by incidental operations of the connecting mechanism aspace corresponding to two or three steps is allowed at the beginning of the series of strips before the first pair of strips is reached. tVe may term this block of non-conducting material and the fixed contacts mounted thereon the switchplate V. A vertical shaft 5- is supported generallyin the centerot' the framework. It is capable of revolving in its bearings and also of vertical or longitudinal motion there in and carries a horizontal arm E, termed herein the select0r-arm. (Shown in the several figures and of which Fig. 8 represents an end view.) The arm E extends radially to a point beyond the outer edge of the insulatingblock and ends above the several fixed terminals u 1:, so that when the shaft 8 is revolved it moves in the arc of the said block over the several fixed points in succession. At its end are two contact-springs insulated from each other, which on the depression of the vertical shaft slide over, clasp, and make electrical contact with the two members of any pairof the fixed multiple contacts above which they may be at the moment when the shaft descends. These springs are made sufficiently self-centering by having their tips bent outward, so as to jointly form a flare adapted to guide the said tips respectively into contact with the outer Sides of the two members of the pair of fixed pointsor strips, as in Fig. 8. The said springs constitute the movable or active terminals cf of the circuit represented by the selector-arm and may be electrically associated with such circuit, one

shoe or are shaped insulated metal plate 13, secured to the block U, and the other, f, through the selector-arm and the metal framework and mechanism of the selector apparatus. The connection of the springe with the curved plate 13 is made only when the selector-arm is depressed and is formed by an insulated conductor 10 and spring 12, carried on the under side of the selector-arm. The pairs of fixed contact-strips of the several other circuits are electrically connected by conductors attached by soldering or otherwise to their ends with their respective substation-circuit conductors and each pair also with similar pairs of fixed contacts represent-. ing the multiple switch-terminals of the same circuit at other selector switch-plates.

The revolving motion required for the circular movement of the selector-arm is imparted to the shaft 5 by the selecting-electromagnet M, which by its armature ct, armature-rod a and pawl p, attached thereto, actuates the ratchet-wheel 91, causing the same to revolve, and by the pinion n carried on the ratchet-wheel axis, and spur-wheel n engaging therewith, to revolve the said shafts. A retracting-spring 71 is provided, which in its appropriate time, operating through a reversely-acting spur-wheel n and pinion n restores the shaft 5 and the selector-arm to their resting or Zero position. To prevent the undesired exercise ofthe retracting mechanism, we provide a retaining-pawl q, positively actuated by the retaining-electromagnet wt and engaging with the teeth. of the ratchet-wheel n.

The selecting and retaining magnets are in series in the same local circuit, the former being a quick-acting magnet arranged or adjusted to respond readily and promptly to rapid successive makes and breaks of the circuit within which it is included, and thus by a step-by-step motion to revolve the shaft 3 and selector-arm in response to and in correspondence with any suitable system of periodic current interruption controlling said circuit. The retainingmagnet, however, is arranged or adjusted by any of the several expedients familiar to those skilled in the art to be sluggish in responding by the re traction of its armature to the momentary cessations of current, and the said armature cannot therefore follow rapid emissions and intermissions of the said current. One said expedient is to provide an electromagnet with long coresaud aweak retracting-spring and another is to shunt said magnet, as described in Oalleys Handbook of Practical Telegraphy, seventh edition, London, 1878, page 288, by a resistance about equaltoits own. We have, however, in practice obtained the required sluggishness by employing a bipolar electromagnet of ordinary construction and dimensions, with the range of its armature so adjusted that the said arrnature in its forward movement is not checked by a limiting-stop, but comes into actual contact with the magnet-poles, and having the retracting-spring thereof'adjusted so loosely. that it can overcome the tendency of the armatu're under these conditions and the residual magnetism thus encouraged to remain in contact with the magnet-poles only after a considerable and well-defined period of current cessation, during which the said spring has exercised a steady retracting force upon said armature. The first current impulse,

therefore, of any series causes the retainingpawl magnet to attract its armature and to bring the retaining-paw] into engagement with the ratchet-wheel, and the said armature once attracted stays so until the circuit is opened and remains open, when it at length falls back. Thus the said pawl acts as a clutch to maintain each step taken by the shafts and selector-arm until the proper time of release. The selecti ng-magnet M has a retractingspring (1 which retracts its armature a on every interruption of current, and the retaining-magnet m has a similar spring which retracts its armature b when the emissions of current cease altogether. The ratchet and spur-gear are so proportioned that for each forward movement of the armature tr and for each corresponding advance of the ratchet-wheel the selector-arm E moves over the switch-plate V a distance just sufficient to carry the terminals effrom one pair of fixed contact-strips to the next.

Longitudinal downward motion is imparted to the shafts by means of the connectingmagnet N, vertically mounted above the uppersurface and at the rear of the switch-plate V, and its armature-lever (l, which extends backward to a fulcrum and then to a forked extension, by means of which it operates the circuit-changers or commutators H and P and extends forward to an end piece d which bears against the upper end of the upright shaft .9. The armature-lever (1 acts to force down the shaft 3 and the arm E carried thereby against the upward-pressing force of the spring d which engages the foot of said shaft. Thus when the selector-arm E has been stepped around the switch-plate V by the action of magnet M to a position where the movable terminals 6 fare opposite the passive terminals u o of the line wanted it can be depressed by magnet N, which is in a second local circuit, enabling the movable terminals of to grasp and make contact with the said fixed strips to o.

In Fig. l the movable terminals e f of circuit L, which for the present may be considered the calling-line, are shown in a position brought about by the movement of the selector-arm in which they may make connection with the appropriate pair of fixed terminals belonging to circuit L and for convenience in drawing a pair of fixed terminals belonging to line L is shown in a similar position with respect to the movable terminals of line L So in Fig. 2 the movable terminals shown are those of line L, while the fixed terminals are those of L but in Fig. 3, whichillnstrates the central-station arrangement of one circuit only, both movable and fixed terminals are those of line L.

H is a key or commutator associated with each line for transposing the relation of the main conductors thereof to those of a second substation-line with which connection is established, and P is a similar but slightly more complex commutator arranged to effectuate certain desirable changes in the several circuits. Although both commutators are operated by the action of the connectingmagnet N, the commutator H is in Fig. 1 shown at a distance therefrom to more clearly indicate its place in the main circuit outside of all of the other central-station mechanisms of said circuit. In Fig. 3 is shown diagrammatically the operative relation of magnetNand its armature-leverd to both commutators P and H, and Figs. 4, 5, and 7 illustrate an arrangement highly satisfactory in practice. The comm utators H and P in this relation both consist of apair of principal contactsprings'and several auxiliary contact-springs, the two principal springs of each being arranged to be acted upon mechanically by the arms (1 d of the rearwardly-projecting extension fork of the armature-lever d, which arms, asbestshown in Fig. 7, furtherseparate the said springs to alter the contacts when the armature is attracted and allow them to resu me their normal condition on the retraction of said armature. All of the commutatorsprings are clamped in a manner well understood to the non-conducting base U In commutator H y g] are the two principal con tact-springs, and 20,21, 22, 23, and 24 are the several auxiliary contact springs or points. Normally contact is established between and 21 and between 22 and 23; but when magnet N is operated the springs y g are.

moved farther apart, thus, as clearly indicated in the drawings, causing the original contacts to be disestablished and forming new ones between y and 20 and between 3 22, and 24.

The commutator P comprises a plurality of circuit-changing contacts grouped in one appliance, so that they may be all worked together and by the same mechanism. The entire appliance involves the two principal springs 25 26, which are acted upon by the fork member cPofarmature-lever d and which impart motion to the several other springs concerned. With these are associated six auxiliary springs or points 27, 28, 29, 30, 31, and 32. Springs 26 and 27 are permanently united, springs 31 and 32 are both connected to ground, and contact normally exists between and 31 and between 29 and 32. When the magnet N operates and attracting its armature actuates this commutator, the normal contacts are broken and new ones are formed between 26 and 28 and between 25 and 27. The circuit arrangement and functions of these comtnutators will hereinafter be explained.

In association with the selector-arm E is another comtnutator or circuit-changer Q. (Shown in the diagram Figs. 1, 2, and 3 and in Fig. 4.) This consists of a movable member 0 two fixed contact-pieces c c, and an actuating cam or finger 0, attached to the vertical shaft. WVhen the selector-arm E is at zero, the finger c presses upon the movable contact 0 holdim it against the fixed point 0 but as soon as the selector-arm is operated and moves away from its zero position the pressure of said finger is removed and the spring 0 movingby its own resiliency, breaks contact with c and establishes conta t with c. The commutator Q, as will presently be seen, is a portion of the mechanism for preventing intrusion by a third circuit upon circuits already engaged.

The connecting-magnet N is vitalized when the subscriber of the line to which it belongs removes his receiving-telephone from its hook-switch and is denergized when the receiver is replaced.

It is to be understood that the system described herein requires and, in fact, has but a single central battery or source of current S, although for the sake of avoiding complexityin the drawings itis shown atseveral places.

W is a tone-producing appliance of any desired character.- It is shown as an appliance for rapidly interrupting the current of a local circuit, and thereby producing, by means of the induction'coil I induced currents in a branch circuit of any line which, seeking to connect itself with a second line, finds such second line already engaged. Under such conditions the induced currents are enabled to actthrough the intervening conductors on the receiver at the substation where the call was originated, producing a musical tone therein which, as will be understood by the subscriber, indicates that the line wanted is busy.

The testing and discriminating relay 7' is wound differentially,having an exciting-helixz and a neutralizing-helix .2 which when energized by appropriate currents act in oppo sition to one another, each tending to neutralize the influence of the other upon the core. Both helices are grount'led at their inner end, and their remaining circuit connections will presently be manifest. The corresponding relay for subscribers circuit L is indicated by r". The relay has two armatures whose levers 2' ,2 work between front and back contact-stops .2 2 and .2, respectively. The relay 0" is, in fact, an electromagnetic switch, and its function is to establish the connection between the substation-circuit to which it belongs and the movable terminals of such circuit carried by the selector-arm thereof, and this is done when the armatures which constitute switch-levers are in their forward or attracted position; but as long as the said armatures remain unattracted they are in contact with their back'stops, armature .2 then forming a part of the intrusionpreventing apparatus and of an incomplete non-intrusion circuit which includes the neutralizing-coil 2' and armature e a portion of the tone-test or engaged signal apparatus. Whenever a line wanted is free, the upper or exciting relay-coil .2 alone carries a current and the armatures work to completetheconnection between the two lines; but when a line wanted is already engaged current flows through the lower or neutralizing coilz also, the relay will not work, the desired conneclion cannot be had, and the engaged signal is transmitted to the calling-substation.

Substation apparains.-Assumin g that all of the main circuits of the exchange are polystation circuits, or may at any time become so by the installation of additional stations, it maybe said that the apparatus at all of the substations of all of the subscribers circuits is alike. At each station there are the usual call-sendii'ig and call-receiving appliances. A is the call-generator, a magnetogenerator developing an alternating current being preferably employed. Bis the polarized call-bell, and 7 a condenser in series with said bell. The bell is grounded on one side and at the other is connected by a branch through the condenser to a particular one of the maincircuit conductors, while the generator,which also has one of its terminals grounded, is normally on open circuit, butis brought into connection when operated with one or the other of said main-circuit conductors as the call is to be sent to a station of the same or of another circuit. The usual transmitting and receiving telephone apparatus and gravityswitch are also etnployed.

T is the receiving instrument, 1 the transmitting instrument, I the int'luction-coil, and O the telephone-switch. The telephone connections are arranged on the standard plan of the central-battery system, the receiver and transmitter being both in circuit with the primary winding j of the inductioncoil and with the condenser 71:, while the secondary winding j is directly in the main circuit. Besides controlling the telephone connection, the switch 0, by means of two separate circuit-breakers o 0 is enabled to disconnect the bell-magnet ground from the main conductors on both sides of the bell when the receiver is taken from the hook for the purpose of preventing the said ground from affecting the selectiotrcircuits. Certain additional and special apparatus is, however, also required. This cotnprises a rhythmical or periodic current-interrupter D and two keys or circuit-controllers, one of which, K, is to be depressed during the operation of the callgenerator to send a call to a substation of anothercircuit when the satne shall have been ,connectedwith the calling circuit through the central station,while the other,K ,is to be operated while sending a call to other stations of the same circuit. Finally, for the purpose of balancing the branch to earth through the bell-magnet and its condenser is from one side of the main circuit We provide at each station a similar branch from the other side of the main circuit to earth, including the impedance-coil i and the condenser 70*.

A form of periodic interrupter D which we have found'convenient is particularly illustrated by Figs. 9 and 10. The inner mechanism is mounted upon the inside of the cover D of the case, and on the outside of said cover are an arm J, rigidly attached to the central shaft 15 and having a handle 16, a numbered andperforated circular plate 1, and a peg e for insertion therein. Within the box is a second arm 17, also rigidly attached to the shaft 15. Upon the extremity of arm 17 is a pawl 18, which engages with a spur-wheel x, loosely mounted upon shaft 15. A spiral spring has one end attached to shaft 15 and the other end to post 19. Apiuion m meshes with the wheel x and turns a star-wheel with which engages the weighted escapement 00 whose oscillations are utilized to separate and unite the contact-points at 00 by the arm 00 hen this instrumentis operated at the substation of any circuit for the selection at the central station of another circuit and for connection therewith, the pin or plug e is first inserted in the numbered hole of the wanted subscribers line, as 10. The arm J is then moved by its handle 16 in a clockwise direction until it is stopped by pin 6 and in moving winds up spring 00 The manipulator thereupon releases the handle 16, and the arm J, impelled by the power thus stored in said spring, returns to zero. Now when the arm J was moved to the pin, as just described, the inner arm 17, rigidly attached to the shaft 15, moved also and through an arc of precisely the same degree as that of arm J. In the movement of arm 17 its pawl 18 slipped backward inefiectively over ten of the teeth of the wheel at, which remained stationary, being steadied and held by the stop-pawl 34. When the shaft begins its contraclockwise rotation, the arm 17 by means of pawl 18 now engages and turns the wheel 00 and also pinion m and star-wheel 00 whose escape x retards and regulates saidreturn movement. The arm 13 secured on the same shaft as the escapement,successively separates and reunites the contact-springs and 36, thereby opening and closing the circuit wherein the said springs are included a number of times corresponding to the hole in which the pin 6 was placedin this instance ten times. The return motion of shaft 15 is stopped by pawl 18 coming against stop 37 and by the arm J striking against post J Wherever the peg e is set, the arm 17 and pawl 18 pick up the spur-gear w and move it througha distance always the same for a given peg-hole, each hole in the plate 1 representing one tooth of the gear 00 and of the pinion m and hence of the star-wheel m, which has the same number of teeth as the pinion The key K has two principal and three an x? iliary contact springs or points. 38 and 39 are the principal ones, and the others are indicated by the numerals 40, 41, and 42, respectively. Of these, 38 is in permanent union with the main conductor 3 and 39 with the ground; but neither is in normal electrical contact with any of the others. The points 41 and 42 are normally in contact. VVhen the keyis depressed, the points 41 and 42 are separated, while contact is made between 38 and and between 39 and 42.

The key K has a movable contact member 43 and two complementary contact-pieces 44 and 45. Normally the points 43 and 44 are in contact; but when the key is depressed the said points are separated and the movable member 43 makes contact with the fixed point 45. Both keys resume their normal condition when they are no longer pressed.

Circuit's, electrical connections, and operation-The electrical arrangement of the several substation-circuits is identical. To describe X will therefore answer for all. In the resting state of the circuit all of the paths through the substation X between the main conductors 2 3 are conductively discontinuous. The branch through the bell B is, however, made receptive to ringing-currents by the interposed condenser 70. This in every case indicated in Fig. 2, which shows the arrangement of two substations) is in an earth branch from conductor 2. The said branch leads from point 46 on the said main conductor 2,'th1'ough the closed resting contacts of the interru pter D and the two keys K K,to the condenser 7a, and from thence through the circuit-breakeroof switch 0 byconductors 47 and 48 and by way of the point- 49 to the bell-magnet B and ground connection 50. To aid in maintaining the circuit balance against inductive disturbance, there is a similar but dummy branch from main conductor 3 at 51, passing through conductors 52 and 53, the circuit-breaker 0 condenser 10 and impedance coili to earth. Passing toward the central station 0 the normal path of the substation-circuit may be traced by conductor 2 to the resting contact 23 22 of commutator H and from thence to the other main conductor 3, through conductor 54 and the bridge 8, extending between points 6 and 7 on the main conductors, respectively, and including the sourceof current S and the impedance-coils I i, and then through the two exciting-coils 10 10 in series of relay R, conductor 55, and the closed resting contact 20 21 of key H. The said source is grounded at its pole nearest to main conductor 2. By fol-lowingt-he extension 56 of conductor 2 from the bridge-point 6 it will be seen that it passes to the multiple fixed terminals or points 'Lb of the circuit, one for the movable terminal of each other circuit. That which is associated with the selector-arm E of substation-circuit L is shown in place; but the others are merely indicated to show their circuit relation. In like manner the fixed terminals 0 of the other side of the circuit are arranged to connect with an extension of main conductor 3, passing from point 7 of the bridge by conductor 57, the normally separated contacts 26 28, conductor 58, commutator-contacts c 0 and conductor 59. The movable terminal 6 of the circuit connects with conductor 3 at contact 24 of key or commutator H after the said key has been operated by way of conductor 60, front and armature contacts a and 2 of relay 7, resistance Rh, conductor 61, and the contacts 24 22, and 1 of said key, and the movable terminalf, representing conductor 2 of the circuit L, in like manner unites with said conductor 2 by way of conductor 62, armature z of relay 7', front contact-point z and conductor 6 3, conductor 57, contacts 27 25 of commutator P, conductor 71, condenser 72 conductor 55, and contacts 20 and 3 of the key H. Thus the said movable terminals or seelectors are normally detached from their circuit-conductors and only become attached thereto when after selection has been made and the said movable terminals have been brought into contact with the fixed terminals it 1* of the wanted line, and when also after the commutator H has been operated the relay 9* becomes excited and attracts its armatures z. The resting-contact 5 of armature 2 is united byconductor 66 through coil 2 of relay 7' to ground, and the resting-contact Z7 of armature .2 connects by conductors 64: and with the apparatus for producing the busy tone-signal currents.

Separable contacts a? a associated with the armature-lever and its forward-limit stop of selecting-magnet M, constitute a short circuit around the exciting-helix 103 of relay R, and when closed permit the sluggish armature 7L of said relay to swing over to the forward pole thereof.

In the commutator P, operated by the vertically-moving selector-shaft s, the contact made between members 25 and 27 thereof supplements the foregoing short circuit and when the shaft descends maintains the same, even though the armature of the selectingmagnet shall have been retracted. The contact made under like conditions between members 26 and 28 of said commutator constitutes a circuit-closer to connect the fixed terminalst'ofalinewiththesourceofcurrent, thus raising the potential of said terminals to furnish a test-current for use in protecting a busy line from the intrusion of a third circuit. The normal contact between members 30 and 31 of the commutator is directly in the circuit (57 of the selecting and retaining pawl magnets, which circuit extends from the battery 3 through the frame and armatureg of relay R, the helices of'said magnets M and m, to and through said contact to ground. Its purposeisto open the said circuit on the operation of the connecting-magnet Nand the consequent depression of shaft s, and the contact normally established between memhers 29 and 32 of commutator P is in an earth branch of the connecting-magnet circuit 68, extending from the grounded battery S and through the frame and armature hot" relay R by the same route as local circuit 67 and continuing through the exciting-coil of connecting-magnetN and the upper Windi-ngz or ex citing-coil of the differential relay 0", the said earth branch and its contact members 29 and 32 being placed intermediately between the magnet N and relay r'to shunt the latter during the first part of the operation of said magnet, thus making the current available for said magnet stronger than it could otherwise be. \Vhen, however, the said magnet has operated and the shafts has been depressed, the contact between 29 and 32 is broken and current is admitted to the relay-coil to excite the said relay.

The commutator Q, which has resting contact between its members a and c, operates as soon as the selector-arm to whichit belongs is moved from zero, and breaking the said resting contact establishes a new one between 0 and a", by means of which a substitute and more direct path for test-current from the source S to the fixed terminal o of a callingline is provided by way of conductor 89, which previously was open at point a, the first path through contacts 26 and 280E commutator P being open during selection.

It will be understood from what has been stated that normally every exchange-circuit is conductively continuous through the battery S and both coils of its own relay R at the 7 central station 0 and conductively discontinuous at the substation, and that consequently by closing it at the substation the relay R may be operated.

in the operation of this system let us as su me that a subscriber-for example, No. 20- 3 of substation X on circuit L-desires to communicate with substation Y of circuit L whose number, we will say, is 37-2. He first removes his receiver Tfrom the switch-hook O and listens to assure himself that his own line is not already in use by some other station. This action 'closes the circuit of the source S at the substation and current flows through both windings of relay Rand through the telephone apparatus at the substation. The preliminary route of this circuit between the two poles of the source is as follows: impedance-coil i, the relay coils 10 and 10 conductor 55, commutator H, main conductor 3, point 67, the induction coil secondary j switch 0, transmitter t, the two substation-keys, the resting contacts of the periodic interrupter D, point 46, main conductor 2, transposing-commutator H, conductor 54, point 6, and impedance-coil I. The

current now flowing is steady and sustained and is consequently responded to by both armatures of relay B. At this moment, however, though the selecting and connecting magnets M and N are. both operated by the closure of their respective circuits 67 and 68 IIO and though the shaft 8 is depressed and operates the commutators H and P, these movements are with one exception incidental and do not involve the exercise of. any useful function. The exception is that by the depression of shaft 8 and the consequent operation of the commutator P the normally separated switch members 26 28 are brought into contact, thereby connecting the ungrounded pole of battery S by way of point 7 on batterybridge 8, conductor 57, contacts 26 28, condnctor 58, resting-contacts c c of commutator Q, and conductor 59 to the multiple fixed terminals 1; of the main circuit, whereby the potential of said terminals is raised to prevent intrusion in the initial stage of the operation by another calling subscriber, as will presently be explained. Moreover, though the selecting-magnet M will in this initial closure actuate its pawl and possibly move its selector-arm one space over the switchplate it will not thus be thrown out of its proper position to begin the selecting operation, for the operation of magnet N and the depression of shaft 8 follow so promptly that the local circuit of magnets M and m is at once opened at the contacts 30 31, and these magnets then lose control of the ratchet-wheel n, whereupon the shaft 3 and selector-arm E, if they have been moved, swing back to zero. Finding the circuit disengaged, subscriber No. 20 at X revolves the arm J of his currentinterrupter D to No. 37, representing the desired line, the pin e having been placed in the hole adjacent on the face of the apparatus to such number, and then releases it. The return of the arm and the revolution of the gear-impelled star-Wheel open and close the main circuit in rapid succession a number of times corresponding to the number of the line wanted, and each time the circuit is closed the lower or quick-acting armature g of relay R responds and is attracted forward, falling away again with each break. The local circuit 67 of the selecting-magnet is by the said armature movement correspondingly made and broken, and the said selecting-magnet M is intermittently excited, the attrac tions and retractions of its armature a copying those of armature g and actuating the selector mechanism, hereinbet'ore described, to move the selector-arm E around the switchplate V until the movable terminals cf of the circuitare directly opposite and above the fixed terminal contact-strips u v of the line wanted-No. 37. The local circuit 67, controlled by the light armature g of relay B, after passing through the winding of the selecting-magnet continues through that of the retaining-pawl magnet on and through the normally united contacts 30 31 to the ground or return connection. The said retainingmagnet (which, as previously explained, is sluggish in its discharge and reaction) attracts its armature b with the first impulse of the periodic series produced by the interrupter, and thus brings'the retainingpawl (1 into engagement with the actuating ratchet-wheel to secure the same and the selector mechanism from falling back during the intermissions of current when the selecting-magnet armature is attracted for a further stroke; but the said armature I), having once moved, is not retracted by said intermis sions, but remains attracted with its pawl in engagement until the connection is completed. In this way the retaining-pawl is brought into engagement with the ratchetwheel and caused to remain there during the work of selection, acting as a clutch to hold each step advanced by the selector shaft and arm. The heavier armature h of relay R, which was attracted by the steady-current when the circuit was first closed, falls back to its normal position, with its rear end close to the rear magnet-pole, as soon as the periodic interruptions of the circuit begin and retains its retracted position as long as the said interruptions continue, the cumulative lag of the armatures g and a, the current in winding 10 of said relay, and the slight oyerbalancingof the armature h cooperating to that end, for although the armature Ct has an associated circuit-closing device comprising the armature-contact (t and fixed contact a and tending to short-circuit the said winding 10 through the conductors 57, 69, 70, and 71 with each pulsation of current the frequency of interruption in the main circuit, and consequently of separation of the points a a is so high that the resistance of the shunt around the winding 10 is too great to drain much of the current therefrom. When, however, the substatioirinterrupter has returned to zero position and selection of the desired line is completed, the current again becomes steady, and the lighter armature g of relay R is steadily attracted and held in its forward position, thus closing the local circuit 67, and thereby causing the'selecting-inagnet M in turn to steadily attract and hold its armature. The short circuit through a a around the winding it; is then effectively established, the exciting-current through the said winding is drained therefrom, andthe heavier and slowacting armature h. no longer held back byits rear pole, is permitted to yield to the influence of the front pole, excited by coil 20 and swings over to a position wherein its front end is brought close to the front pole of the magnet. The circuit 68 of the connecting-magnet N is closed by the forward attraction of armature h, and a current flows from the ungrounded terminal of the battery S to the frame of relayR and contacts of armat u re 71 and through the local conductor US'and connecting-magnet N, returning to the other pole of the battery at this stage of the operation through the contact 29 32 of commutator P. The connecting-magnet is thereby actuated and operates to depress the shafts and selector-arm E, and the movable terminals 6 fof the circuit,which by the selecting process have been moved to a point immediately above the as- ICC sociated fixed terminals n v of the line wanted, are thus pressed down upon and grasp the said fixed terminal strips, respectively, establishing contact therewith. Moreover, the depression of the shaft 3 by the connecting magnet N also operates the keys or com mutators H and P. The operation of the key-P dissolves the contact normally existing between its members 3031 and 29 32 and establishes new ones between 26 and 28 and between 25 and 27. The separation of 30 and 31 opens the circuit containing the selecting and retaining magnets and permits these magnets to release their armatures, so that the actuating-pawl p and retaining-pawl q are both disengaged from the ratchetwheel n. The selector-arm E does not, however, now under the influence of the recoveryspring a, return to its starting-point or zero position, being held in place by the movable contact-terminals 0]", which clasp the fixed terminal strips it 'u of the wanted line. The separation of 29 and 32 removes the intermediate ground connection or cut-off from the local circuit of the connecting-magnet N. The object of this initial ground connection through contact-points 29 32 was to reduce the resistance of this circuit by short-circuiting the upper winding or exciting-coil of the differential relay 1", thus producing maximum current in the local circuit 68 for the operation of magnet N, while the latter is performing the work of forcing the shafts down; but the short circuit being now removed the said relay-windingz is introduced into the circuit 68 in series with the connecting-magnet N. The contact established between members 25 and 27 of commutator P closes a new short circuit around winding 20 of relay R, that through the circuit-closing points 01 a being opened by the disexcitement of magnet M pursuant to the opening of its circuit at the comm utator-contacts 30 31, and since this new short circuit remains intact as long as the communication between the calling and called line continues it is evident that the winding Q03 of relay R is excluded from the talking-circuit duringconversation, and the contact established between members 26 and 28 of commutator P, which, on original depression of the shaft 8 supplied a testing po tential through contacts 0 c of commutator Q to the fixed terminals -v of the calling-line, is of no consequence at this point of the operation, for the reason that the movable member 0 of said commutator Q was detached from its contact with c on the first rotary movement of shaft .9, the said movable member being suffered to make contact with the back contact c, and thus to form a circuit-closer act ing to establish a testing potential in connection with fixed terminals'v, but under different conditions, whereby the line is guarded from intrusion during the time required by the selecting process and subsequent thereto. We have seen that the connection between the movable terminals 6 f of the calling-circuit and the fixed terminals it v of the wanted circuit was effected immediately upon the depression of shaft 5 by the connecting-magnet N. This operation, however, does not establish communication between the said two circuits, for the reason that the said movable terminals are normally detached from the main circuit they represent. Tracing back from the said terminals, it will be found that neither of them leads directly to its own circuit-conductor and that to attach them to their respective conductors it is essential that the relay 0* shall attract its armatures. We have also seen that the depression of shaft s was followed by the introduction of winding 2 of relay 0' into circuit 68, so that the cur rent flowing through such circuit would also flow through said winding. If a substantially equal opposing current should now How through the other coil .2 of the relay, the said relay will remain inoperative and the attachment of the movable terminals to their circuit would not be eifected, and this is, in fact, what occurs if the line wanted be already busy; but we have assumed the said line to bedisengaged and thatacurrentflows through coil only. The circuit of said coil beginning at the battery S is traceable through conductor G7, the metal frame of relay R, armature 7t, connecting-magnet N, conductor 68, and through winding .2 of relay r to ground. The relay 1' is excited and attracts its armatures a .2 to their forward positions, establishing connection between the main conductors 3 and 2 and the movable terminals 6 and f, respectively. At this juncture the main circuit L of subscriber No. 20 is connected by its two movable terminals with the corresponding pair of fixed terminals of the main circuit L of subscriber No. 37; but the talking-circuit between the two substations X and Y remains uncompleted until the said wanted subscriber No. 37 lifts his receiver from his switch-hook; There is, however, a single-conductor earth-return circuit between the said stations over which call-signals may be transmitted.

To ring or send a call from station X to station Y after the selecting and connecting operations described, the subscriber at X actuates his call-generator A, at the same time pressing the key K. The generator may be fitted with the usual automatic controlling device to close its circuit and to connect itself with the line by uniting the points 73 75, the connection in this instance being made through the intermediate piece 74, a branch 77 being led therefrom to earth through the home bell, so that a ring may be produced at the transmitting as Well as at the receiving station. The circuit through which the ringing-currents developed by the generator reach the bell at Y is particularly shown by Fig. 3 and is as follows: from the grounded callgenerator A through conductor 76, contacts 40 38 of key K, conductor '78, main conductor 3, contacts if, 22,

and 24: of commutator H,

(which, it is to be remembered, has been op a of differential relay 7', conductor (30, movselected multiple contacts 10 o.

able terminal 6 of selector-arm E, fixed terininaltt of substation-circuitL conductor 79, contacts 22 23 of commutator 1-1 (which has not been operated to transpose its several couductors,) main conductor 4 of circuit L and at the substation Y from said conductor at 46 through the interrupter D, the restingcontacts 43 44 and 42 41 of the keys K and K, respectively, and the condenser to the bell B and ground. These appliances and connections at station Y are not shown; but the arrangement is exactly the same as at station X. At the same time a second ground return-circuit is closed by the ringing-key K through the forward exciting-coil 10 of relay R, maintaining the attraction of armature h of said relay, and thereby, through the intermediation of connecting magnet N, maintaining the depression of shaft 3 and the engagement of the selectorsprings ef with the This circuit leading from the ground connection of said key continues by way of contact-points 39 42,

the resting-contacts of key K conductor 81, resting-contacts 00 0f interrupter D, main conductor- 2, closed contacts y 20 of commutator H, conductor 55, winding tu of relay R, the shunt-circuit 71 57 of the winding w of said relay, point 7 of battery-bridge 8, and through the impedance-coilt'and battery S to ground. This effectively prevents any undesired motion or trembling of the selector-arm while the generator at substation X is being operated and also frees the other main conductor 3 and its continuation 61 from the batterycurrent.

The commutatorH of line L has it two principal contact-springs y and g always electrically united to the main conductors 2 and 3, respectively, of said line. Conductor 2, however,normally connects through the other points or springs 23 and 22 with the multiple fixed terminals 16 of its own main circuit and with the grounded pole of the battery S, while conductor 3 in like manner connects through points or springs 21 and 20 with the multiple fixed terminals 1; of its own circuit and with the other pole of battery S, although said connection is normally broken at the contacts 26 28 of commutator P. When the commutator His operated, the contacts 20 21 and 22 23 are disestablished and new ones are formed of spring 3 representing main conductor 2 with point 20, and of spring 11 representing conductor 3 with points 22 and 24. This can only occur when the connecting-magnet N of the line in question is operated and attracts its armature (I, one of the branches of the forked rearward extension of the lever (1 then acting to push farther apart the springs y and thus produce the abovedescribed changes. The effect of this opera tion is that the main conductor 2 of circuit L instead of being switched to the corresponding conductor 4 of circuit L will be connected to conductor 5 and that conductor 3 of circuit L will be united to conductor 4 of circuit L Now the bells B at the two substations X X of line L are both, Fig. 2, con nected with condensers in earth branches of main conductor 2; but the call-generatorsA at the same substations are in normally open earth branches,which may be connected with either main conductor. Moreover, such also is the case for all of the main circuits of the system L for example, having its substation-bells similarly connected to the particular main conductor 4, which in that circuit corresponds to the said conductor 2 of circuit L, while in each case by manipulating the key K the generator will be connected with the other main conductor. It follows that when pursuant to the establishment of connection between any two lines L and L the commutator H of the connection-actuating line is operated the conductor 3 of the said line with which the call-generator A at substation X is associated by key K is united to conductor 4 of the wanted line with which the call-bells of the said wanted lineare associated. Thus by the transposition of conductors effectuated by com mutator H the subscriber is enabled to ring the bells of the wanted line without ringing any of the bells at other stations of his own line. A suitable code of signals is ofcourse to be employed, and though all stations of the wanted circuit hear the ringing each will answer only to its own code-signal. When subscriber No. 37 atY answers by removing his receiver from the hook, his main circuit is closed and his relay R becomes active, causing the connecting-magnet of that line also to depress its selector-shaft s and to operate its com mutators H P. No selection having previously been made,the movable terminal springs of line L do not engage any fixed terminals of any line. Therefore the connecting-magnet of acalled line acts merely as a means of actuating the circuit-changers H P the former incidentally transposing the line conductors and the latter making the.

several changes of the circuits, which,'in reference to circuit L, have already been described, only one of which, however, is of importance-namely, the shortcircuiting of winding w of relay R that it may not be an impedance in the talking-circuit. Since the operation of calling was previously performed, the incidental transposition effected by H is without significance. The complete talkingcircuit between X and Y is thus fully established, and its character and arrangement are wellindicated by the diagram Fig. 2, wherein the battery S is shown as being bridged at the junction of the two component circuits L L to supply current through the'impedance-coils I 1'1 7? to each independently for a transmitter-supply, while the voice-currents developed which the voicecurrents easily pass.

More in detail, the talking-circuit may be thus traced: from secondary winding of transmitter induction-coil I at station X of circuit L and passing through branch cond uctor 83, main conductor 3 to springy and contacts 22 2t of commutator H, conductor 61, resistance Rh, armature of relay 7', front stop .2 thereof, conductor 60, movable terminal or select-or spring 6, fixed multiple contact u, of line L conductor 79, contacts 22 g of commutator H of line L and main cond uctor 5 of that circuit to substation Y, and at such station through the secondary of the transmitteriru'luction coil,theswitclrhook O, the transmitter, the keys K K and the interrupter, thence returning by way of main conductor 4, contacts y 20 of commutator H conductor 8-1, the winding 0f relay R and its shuntingcondenser 7L2, conductor 85, contacts 25 27 and 26 28 of commutator P conductor 86, resting-contacts c c of the testcurrent commutator Q conductor 87, fixed terminal t of line La'movable terminal spring f of line L, conductor (32, armature .6 and.

front contact .2 of relay 1", conductor 63, shunt-conductors 57 and 71, winding 20 of relay R and its shunting-condenser, conductor 55, contacts 20 and y of commutator ll, main conductor 2 of circuit L, and reaching substation, X through the interrupter D, the resting-contacts of keys K and K and transmitter 6 back to the secondary coil of said transmitter. The two stations are now in full communication and may converse. At the close of the communication both subscribers replace their receivers on theirrespectiveswitch-hooks. VVhenXhangs his receiver on the hook, the relay R of line L releases its armatures g and h, causing the connecting-magnet to relax its pressure on the shaft .5', which thereupon rises, being pressed upward by the spring (I beneath its heel. As soon as the selector-springs cf are raised above the fixed terminal strips u t), with which they have been in engagement, there is nothing to hold the selector-arm, and, impelled by the recovery-springn ,it promptly swings back to itsinitial position and is ready for another call. Of course the normal condition of the commutators H, P, and Q is also brought about bythe return vertical and circular motions of the shaft 3 and arm E. In like manner the replacementof the receiver at Y causes relay R t o lose its excitement and to release its armatures, whereupon shaft 8 of line L is elevated, restoringthe initial condition of commutators H and P and this line. also is now ready for further business.

We shall now describe the operations Whereby intrusion upon or interruption of engaged lines is prevented and whereby a subscriber desiring to connect his circuit with an engaged linemay be informed that it is already occupied. The differential relay 7* of each line, the movable terminal f, united with the lower armature .2 of said relay, the commutator Q, the contact members 26 28 of commutator P, and the musical-tone signal apparatus W', with its induction-coil I are concerned in these operations.

From what has been stated it is manifest that the armatu res of the differential testing or discriminating relay 0' must be attracted into their forward positions before the connection between a calling and a Wanted substation-circuit can be completed. If currents of appropriate strength and direction pass through both coils of the differential relay,the said relay remains quiescent and will not attract its armatu res into the said necessary position. Gonverselydfacurrentofappropriate strength passes through the upper coil only, the said relay becomes active and does at tract. its armatures. Since the lower relay coil .2 is in direct connection with the lower armature. .2 and conductor 62 with the movableterminalfof its own line, in order to forestall or prevent intrusion it is only necessary to provide that the moment any line becomes busy all of its fixed terminals shall be brought into connection with the ungrounded. pole of the battery. Then when any third circuit endeavors to connect itself with the said busy line a circuit for the battery-current will be formed as soon as the movable terminal of the seeking line touches the fixed terminal of the wanted line, and as this circuit leads through the lower coil .2 of therelayr of the third line endeavoringto establish connection it is clear that the said relay ofsuch line will remain unoperated. The third line accordingly is unable to connect itself with the line desired. Thus the mere act of lifting a receiver from its hook at any substation at once establishes a battery potential at the fixed terminals of the line of such substation, and it is no longer in the power of any other line to connect with that circuit, forthis act closes the main circuit and operates relay R, which in turn act-uates the connecting-magnet N, causing the depression of shaft 3 and the operation of com mutator P, and when performed at the substation which is making the call it is therefore effective to protect the line of such station from intrusion when at the outset of the operation the attendant removes the receiver from its support before making selection.

battery by conductor 57, circuit-closing members 26 28 of commutator P, conductor 58, contacts 0 c of commutator Q, and Wire 59, thence continuing to all of the fixed terminal points '0 of the circuit. The points 12 may be regarded as busy-test points; but in the case of a subscribers line which has called for and has proceeded, to select another circuit this testingcircuit is insufficient, for during the act of step-by-step selection the connecting-magnet is not excited, and therefore the contact members 26 28 of the key P are separated, and, secondly, even though a way should be found to unite these contacts the operation of testing bya third subscriber would tend to interfere with the proper operation of relay R of the calling-line, for the test-circuit of the intruding subscriber forming a low-resistance shunt about the said relay is liable to withdraw current therefrom to such an extent as to make its work in selection untrustworthy. To avoid these difficulties and provide protection from intrusion from the begin ning of the operation concerned in a communication and throughout until disconnection, we unite the fixed terminal testpoints of the call-originating subscriber to the battery through a circuit connection independent of the circuit of his relay and talkingcircuit and connecting with said battery at a point where the potential is always stable. This, as previously explained, is done by meansofthe commutator orcircuit-changer Q, which on the first step ofthe selecting mechanism changes the test-circuitof the calling-line from conductors 57 and 58 to more direct conductor 8!). Should a third subscriber undertake to establish communication with a circuitsay L-which is selecting or has selected and connected itself with another, the following operation would take place: The fixed testing-terminals i) of L have had the potential of the battery imparted to them, as indicated above, and when the movable terminal or selecting-spring f of the intruding subscribers line descends and engages with the associated point e of circuit L the incomplete test and intrusion-preventing circuit of the said third circuit is closed, the differential relay thereof (whose lower or neutralizing coil is in such circuit) refuses to work, and the subscriber desiring to connect'with L is unable to do so. The intrusion-preventing circuit when completed is as follows: from the battery by conductor 89 to members 0 c of commutator Q and by conductor 59 to points 1) of the engaged line L, then by movable terminal fof the interfering line desiring communication with L to armature .2, back contact a and lower coil .2 of the differential relay 1" thereof completing the circuit to ground. As current is at the same time also flowing through the upper coil of said relay, the two coils oppose one another, the relay remains quiescent, and the connection cannot be secured.

In the case of desiring to communicate with a called engaged subscribers circuit the outcome of the testing operation is the same and is reached generally in the same way, it being essential in this case also that the said circuit shall be protected from intrusion from the moment that it becomes concerned in a connection. This occurs as soon as its passive terminal 12 is touched by the active terminalfof the original calling-circuit, for f is and consequently 1; becomes conductively united to the ungrounded battery pole by way of conductor 62, armature 2 and front stop Z6 of relay r, conductor 63, and point 7. It is therefore the action or non-action of the differential relay 4" of a line seeking to unite itself with another that determines whether or not such union shall take place, and the said action or non-action of said relay depends upon the engaged or disengaged condition of the desired line. If the said line be disengaged, the relay of the seeking line becomes operative and effectuates the union desired by uniting the main conductors of its own circuit with their normally detached movable terminals; butif theline wanted be already in use the relay of the seeking line remains unexcited, because'the incomplete non intrusion circuit has been completed through the neutralizing-coil of the relay and the battery attached to the terminalsc of the wanted line and fails to establish such union. At the same time if a wanted line is busy a tone-signal indicating that fact is transmitted over the seeking line and may be heard in thereceiver of the substation of said line. This is produced by the constant operation of the tone-producing apparatus W, which rapidly interrupts the circuit of the source of current and by means of the induction-coil I transmits the said tone-producing current to any line-circuit connected through conductor to the secondary of said coil. The operation of the commutator H of any line connects through contact 24 the main conductor 3 or 5 thereof with the said tone-test conductor ('55, provided the upper armature 0r switch-lever 2 of the differential relay remains on its resting-contact .2 as we have seen is the case if the line wanted be busy.

The complete circuitof the busy-signal current from the tone-producing apparatus to the main conductor of any individual substation and thence through the substation-receiver is traceable by conductors 65 and 64, fixed and armature-contacts a a of the differential relay 'r, conductor 61, including resistance Rh, contacts 24, 22, and 3 of commutator H to main conductor 3, point 67 thereof at the substation, conductor 83, induction-coil windings j andj when united by the switch-lever, receiver T, condenser 70, and through the key K and interrupter D to main conductor 2 of the substation-circuit at 46. From this point the tone-test circuit continues itself to ground mainly by the line back to the central station, through contacts y and 20 of key H, conductor 55, relay-coil and condensers w and 72 too 

